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At the same time it possesses a totally distinct series of functional gills, which are not derived from the modification of the typical molluscan ctenidium.

The left ctenidium or gill-plume. vent, Ventricle of the heart. a.b.v,

Each gill has the structure of a typical molluscan ctenidium, consisting of an axis bearing an anterior and posterior row of filaments or lamellae.

Spiral fibre: the spiral thickening or folding of the chitinous lining of a trachea, which gives to the latter its characteristic microscopic appearance as well as its support and elasticity:= ctenidium.

Cephalic shield continuous with dorsal integument; no shell; ctenidium projecting from mantle cavity.